Electrohydrodynamic generator with collector extension

ABSTRACT

In an electrohydrodynamic generator, a collector extension covers the collector/injector interspace region and prevents radial dispersion of charge carriers. The collector extension is tubular leaving the main body of the collector and opens into a wide mouth at the top of the injector assembly. The mouth is even with or extends past the top of the injector assembly.

P1538502 QR 3 636 388 United States Patent [151 3,636,388

Seeker et al. [4 1 Jan. 18, 1972 54] ELECTROHYDRODYNAMIC 636,304 11 1899Vosmaer ..s5/1s2 x GENERATOR WITH COLLECTOR FOREIGN PATENTS 0RAPPLICATIONS EXTENSION 371,599 12/1921 Germany ..ss/150 21 lnvenwrsrPhilip Secker, AngleseyNoflh Wales; 380,917 9/1922 Germany ..ss/150 J hn1123 1991 1959? C9 9 Y 1 [73] Assignee: KDI Corporation [22] Filed: June25, 1970 [2!] Appl. No.: 49,723

Primary Examiner- B. X. Sliney Attorney-Sughrue, Rothwell, Mion, Zinn &Macpeak [57] ABSTRACT In an electrohydrodynamic generator, a collectorextension covers the collector/injector interspace region and prevents[52] US. Cl ..3l0/10,255/152 radial dispersion of charge carriers. Thecollector extension is [51] Int. Cl. 512 tubular leaving the main bodyofthe collector and opens into a [58] Field of Search l wide mouth atthe top of the injector assembly. The mouth is l 56] References Citedeven with or extends past the top of the injector assembly.

UNrrisbS'rATEs PATENTS 4 Claims, 7 Drawing gum PATENTED JAM 8 l9?! SHEET1 OF 3 INVENTORS PHILIP E. SECKER JOHN F HUGHES ATTORNEYS Pmmmmwmzafsselaaa SHEEI 2 OF 3 PATENTEU .mu 8 m2 SHEEI 3 [If 3 A CL 2 mm m T RYTCT R D EC O ED 4 4 MW .m m WU 0 4 m m. III/l WALLS.ELECTROI-IYDRODYNAMIC GENERATOR WITH COLLECTOR EXTENSION BACKGROUND OFTHE INVENTION electrostatic generators.

2. Description of the Prior Art Recent developments have indicated thatflowing liquid electrostatic generators, referred to aselectrohydrodynamic (EHD) devices, can meet the need for electrostaticdevices capable of producing voltages within the range 500 kv. to 2 mv.Such units could be ,used for E.H.V. electron microscopes and for use insemiconductor or integrated circuit ion implantation service.

The operation of flowing liquid electrohydrodynamic (EHD) devices isbased upon the fact that it has been found to be possible to injectsignificant currents into dielectric liquids, e.g., hexane, in anondestructive manner by using an injection system which includesfield-emitting electrodes. In its'most basic form, the injection systemconsists of a razor blade, serving asthe injecting or emittingelectrode, mounted perpendicularly to a flat wire mesh grid, serving asa'positively biased counter electrode. Application of a voltage of aboutkv. to this diode provides, due to the intense field emission, anegative space-charge sheath around the razor edge. The electrons whichare emitted from the metal surface of the razor are trapped by liquidmolecules in the dielectric to form negative low mobility chargecarriers. These charge carriers move towards the grid or counterelectrode under the action of'the electric field.

Collisions between charge carriers and neutral molecules in the bulkliquid result in momentum transferpClose to the grid, momentum transferfrom the mechanically drivenbulk liquid to the charge carriers sweepssome of the charge carriers beyond the grid wires and into a field-freeregion beyond the rid. g A simple EHD generator consists of a chargeinjector to produce charge carriers, a means for moving the liquid toenable the charge carriers to be driven into a field-free region,

and a means for stripping the charge from the liquid.

FIG. 1 of the drawings illustrates a typical prior art EHD generator insimplified form. With reference to FIG. 1, the razor blades or injectoror emitter electrodes are denoted by numeral 102, a flat wire mesh gridor counter electrode bynumeral 104, the collector by numeral 106,holding brackets by numeral 112, a conduit for liquid flow by numeral114, and the grid holder by numeral 1 16.

In such a system, the flowing liquid is returned to the emitter 102after passage through the collector 106 by a return conduit not shown inFIG. 1. The return conduit may be formed of glass or a plastic, such asunplasticized polyvinyl chloride. A circulating pump to mechanicallydrive the liquid through the system is usually incorporated in theclosed loop. For hexane, fluid velocities in the injector have been ashigh as 0.5 m./s. High fluid velocities significantly enhance theinjector current. This is believed to be the result of space-chargeremoval from the emitting edges of the injecting device (razor blades).

Each of the three razor blade emitters 102 typically has an edge lengthof 12 mm. and a tip radius of approximately 1,250 A. With a suitablepotential difference between the emitters 102 and the grid 104,field-controlled emission occurs into the flowing liquid dielectric.Rapid field divergence away from the emitter edge insures that liquidionization and deterioration is small. Because of the small radius ofthe razor blade tips, an applied voltage of only a few kv. results inintense electric field and copius electron emission. This provides thehigh density of negative charge carriers in the dielectric liquid, andthe region about the injector can be considered a quasivirtual cathode.The space charge, due to the negative charge carriers in this region,reduces the field at the razor blade edges so that in the steady stateemission is reduced to that level required to offset the charge loss byconduction from the virtualcathode region. Emitter 102-grid 104 spacingsin the prior art have typically been 1-2 mm., providing an emitter fieldof approximately 10" to 10' v.cm.l.

The magnitude of current resulting from the application of an adequatevoltage between the emitters l02 and the grid 104 is basicallyproportional to the product of the number-of emitters and the totalcarrier velocity. If. adequate separation between the razor bladeemitters 102 is provided so thateach emitter can operate in aspace-chargelimited mode, multiple blade arrays in excess of threecan beused to provide substantial injector currents. For instance, the priorart obtained a maximum injector current of 550A..utilizing 43 bladeswith a tip-grid spacing of 1 mm. and an applied voltage of 9.5 kv.

The prior art has also used multiple-tipped corona emitters and sharpline emitters. For the latter system, ionization was increased by usingan easily dissociable additive in N-octane. Chemical degradation of theliquid wasencountered in such systems.

The grid 104 may consist of a fine mesh grid of 30 lines per inch andthecollector may be cylindrical brass plug. The grid is required toestablish the field necessary to create negatively charged carriers inthe liquid and is positively biased with respect to the emitter. Themeshed shape of the grid and the fast flowing liquid help to carry thecharge carriers past the grid to the collector where they are scavengedor collected.

Instead of using separate assemblies for emitter and grid electrodes, itis also known to use a single injector assembly which includes acentrallyplaced cylindrically shaped counter electrode (acting similarto the grid) and a plurality of emitters or injecting electrodes (razorblades) placed circumferentially around the counter electrode, with theliquid flow occurring through the injector assembly to sweep away thecharge carriers created thereby.

Once an EHD generator, as'described, is in operation, the potential onthe collector builds up relative to the injection system. The associatedelectric field exerts a repelling effect on further charge carriersmoving towards the collector. At some point, this repelling force on thecharge carriers reaches a degree where the charge carriers becomestalled between the injector and the collector due to the collectorrepelling field nearly balancing out the momentum transfer effect of theflowing liquid. At this point, where axial carrier motion becomesgreatly reduced, the charge carriers which are essentially stationaryare prone to radial diffusion as a result of their mutually repulsivefield. This effect results in serious charging of any insulating wallsbounding the conversion space. This can lead to disruptive flashover toany grounded surface. Furthermore, device efficiency is seriouslylowered since charge carriers are effectively lost from the collector.The present invention solves both of these problems.

One solution to the problem of radial charge carrier dispersion isdisclosed and claimed in the copending patent application of P. E.Seeker, entitled Improved Flowing Liquid Electrostatic Generators, filedMar. 23, 1970, and having Ser. No. 21,742. The solution describedtherein includes the creation of a sheath of uncharged liquidsurrounding and traveling parallel to the charged liquid in theinjector/collector interspace region. The injector assembly containssurrounding inlet/outlet ports which allow a part of the liquid to passtherethrough unaffected by the injector charge emission.

As will be appreciated by one skilled in the art, the theory andpractice of EI-ID generators is well documented, and the applicantsaccordingly wish to incorporate, by reference, the followingpublications which discuss the operation of EHD generators:

liquid-Filled Electrostatic Generator," Electronic Letters,

Vol. 2, No.5, May 1966, J. F. Hughes and P. E. Seeker; High-currentInjection Into Liquid Hexane Using Field Emitters," Brit. J. Appl.Phys., Vol. 17, pp. 885-890, 1966, G. Coe, .I. F. Hughes and P. E.Secker;

Collection of Charge From a Moving Stream of Liquid Dielectric, Brit. J.Appl. Phys, (.1. Phys. 1).), Ser. 2, Vol. 2. 1969, J. F. Hughes and P.E. Seeker;

SUMMARY OF THE INVENTION Substantially all radial charge carrierdispersion in the injector/collector interspace region is eliminated bythe present invention.

A collector extension down to and partially surrounding the injectorassembly results in an electric field in the region of the injectorassembly having lines of force directed inwardly. The lines of forcetend to push back the charge carriers which would otherwise be radiallydispersed to the outer walls of the generator conduit. The collectorextension enclosing the injector/collector interspace is bell shaped orbottle shaped to result in the desired lines of force. The injectorassembly is mounted on a platform which is movable up or down to varythe relative position of the injector assembly and the mouth of thecollector extension to achieve optimum voltage generation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of aprior art EHD generator;

FIG. 2 is a cross-sectional view of a preferred embodiment of thepresent invention;

FIG. 3 is a top view of the preferred injector assembly used inconnection with the embodiment shown in FIG. 2;

FIG. 4 is a cross-sectional view of the injector assembly of FIG. 3taken along lines 44 ofFIG. 3;

FIG. 5 is a front view of a blade holder assembly which forms a partofthe injector assembly of FIG. 3;

FIG. 6 is a perspective view of the blade holder assembly; and

FIG. 7 is a schematic diagram of a part of the injector/collectorassemblies and illustrates the lines of force which are generated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The broad theory of EHDgenerators has been heretofore outlined in the description of the priorart. The following discussion is offered to set the context of thepresent invention in systems which have proven most efficient. It willbe appreciated by one skilled in the art that the practice of thepresent invention can find application in systems as the prior art hasutilized, and in systems which will vary from those described in thespecification. In the preferred embodiment of this invention, hexane isutilized as the liquid dielectric material. Other materials, such asfreon, etc., could be used. It is of primary importance that thefollowing criteria be met for the liquid dielectric, rather than any onespecial dielectric be selected. Specifically, the dielectric must havegood insulating properties. Hexane, when purified, has a dielectricstrength exceeding 10 v./cm. The liquid dielectric must further have alow negative carrier mobility, a low viscosity and preferably benonfiammable. Typical viscosity values are equal to or less than 0.5centistokes. Any liquid with the above criteria could be used in the EHDgenerator of this invention.

The negative charge carrier mobility for hexane is only l.5X10 cm./v./sec. Negative charge carrier mobilities of this magnitude serveexcellently in an EHD generator. With typical injector designs, theunipolar charge current density in the conversion space with hexane willbe approximately 1.5 uAJcmF. It is possible that the prior artdielectrics, such as gaseous, gaseous-solid systems and the like couldbe used in this present invention. For instance, the prior art utilizeddust particles, aerosols, glass beads, water droplets in a gas, andmetal or gas bubbles in the dielectric liquid. Problems were encounteredwith the use of such carriers, such as accumulation of the chargecarriers in regions of high electric field resulting in electricalbreakdown.

As illustrated in FIG. 2, a preferred embodiment of the EHD machinecomprises a generator portion 40, conduits i0, 20 and 21 for carryingliquid to and from the generator portion 40, a heat exchanger 14 havinginlet tube 16 for carrying water, mechanical pump 22, and bellows l2 and24.

The heat exchanger 14 is included to control the temperature of thedielectric liquid, preferably purified hexane, which flows through thesystem. It has been found that by increasing the temperature of thedielectric by approximately [0 C. a pressure of 7-8 p.s.i. develops inthe system and inhibits bubble formation in those areas in which drasticchanges in cross sectional flow area occur. A further advantage ofoperation at elevated temperature is that moisture condensation onvarious external parts of the system is minimized.

The mechanical pump 22 sweeps the liquid out of the collector region ofthe generator portion 40 and into the injector area of the generatorportion 40. The flow of the liquid dielectric is in the directionindicated by arrows 41, 42, 44, 46, 48 and 50. The bellows l2 and 24,which may be Teflon bellows, are provided to isolate the generatorportion 40 from any vibration of mechanical pump 22.

The generator portion 40 includes an injector assembly 32 mounted on anelevator mechanism 30 for raising or lowering the injector assembly. Theelevator mechanism is shown only by way of example as including a motor31 and gear means 33. The elevator mechanism should include openingstherethrough for passage of the uncharged liquid dielectric in thedirection of arrow 48. Other than fulfilling the latter requirement,just about any elevator mechanism would be suitable.

The generator portion 40 also includes a collector electrode assemblycomprising a main portion 34, an outlet portion 38 and a collectorextension 36 having a mouth which extends a least level with the top ofthe injector assembly 32 and preferably slightly below.

As is known in the prior art, the liquid dielectric passes into theinjector assembly 32, emerging as a charged liquid dielectric. Thecharged liquid dielectric passes into, through, and out of the collectorassembly emerging again as an uncharged dielectric liquid. Thecollection or scavenging of charges by the collector is enhanced byincreasing the ratio of surface area/volume of collector. Increasedscavenging has been accomplished by cutting up old automobile radiatorsand stuffing them into the hollow cylindrical portion of the collector34, thereby greatly increasing the surface area of the collectorelectrode. The metal used to make the collector should preferably beeasily cleaned and relatively slow to oxidize. Copper has been foundsuitable although not the only metal suitable.

The active portion of the generator is held away from the in- Sulatingwalls 26 by means of insulating platform 43. Electrical connections, notshown, are made in a known manner to the emitter and counter electrodesof the injector assembly 32 and the collector electrode.

The manner in which the invention operates to prevent radial chargedispersion can best be understood by referring to FIG. 7 wherein thelines of force, due to the electrical field between the collectorextension 702 and the injector assembly 700, are designated by thenumeral 704. The liquid flowing out of the injector is shown at 706 andthe insulating walls of the generator portion are shown at 708.

As is well known, in flowing liquid electrostatic generators, momentumtransfer from the liquid molecules, as the liquid passes throughinjector assembly 700, to the unipolar charge carriers, generated by theinjector assembly, causes the charge carriers to migrate from theinjector assembly 700 to the collector electrode. Consequently, thecharge is moving counter to the collector/injector electric field. Nearthe maximum output voltage limit the carriers are quasistalled betweeninjector and collector and previously would undergo radial diffusion asa result of their mutual repulsive field. Such behavior gives rise tothe loss of charge carriers from the collector and also can causecharging up of the insulating sidewalls and subsequent flashover toground.

By suitably shaping the entry port (collector extension 702) of thecollector electrode as illustrated in FIGS. 7 and 2, acollector/injector electric field distribution can be established whichboth aids axial carrier collimation and positively prevents all carrierescape from the injector/collector region. The electric field set up bythe potential difference between the collector and injector has aninward radial component to counteract the mutual repulsion field betweenthe carriers. Should a carrier diffuse from the central flowing core ofliquid, it moves into a region where the liquid is almost stationary andthe electric field returns the carrier to the injector. Theinjector/collector system thus constitutes an electrostatic bottle fromwhich carriers cannot escape to set up undesirable insulation chargingeffects.

A preferred embodiment of a novel injector assembly which is suitablefor use with the present invention is illustrated in FIGS..3 through 6.The assembly is cylindrically shaped and includes a counter electrode upof center post 300 and surrounding rings 314 and 318, together with anemitter electrode comprising 21 razor blades 310 having a total of 42emitting edges. It will be noted that the novel injector assemblyillustrated herein is designed to utilize both edges of each razor bladeas emitting or injecting surfaces, whereas the injector assemblydescribed in the copending application mentioned above uses only asingle edge of each blade.

The assembly comprises four concentric cylindrical rings 312, 314, 316and 318. Center post 300 as well as rings 314 and 318, may beconstructed of stainless steel and rings 312 and 316 may be constructedof an insulating material, preferably polytetrafluoroethylene (PTFE).All four concentric rings are structurally held together and to centerpost 300 by means of upper and lower spider members 320 and 322,respectively, which may also be constructed of stainless steel. Spider320 is flat and is held to center post 300 and to all of the rings bymeans of screws 324. Spider member 322 comprises three separaterectangular plates welded to rings 314 and 318 and to center post 300. Aside view of spider 322 is shown in FIG. 4.

Each blade 310 is held by a blade holder 330 which may be constructed ofPTFE or other easily machinable plastic. There are 21 blade holdersshown in the embodiment and they are evenly spaced around the peripheryof rings 312 and 316. Each blade holder is fitted into slots madetherefor in rings 312 and 316. The slots are illustrated by the dottedline 500 shown in FIG. 5. As shown in FIGS. 5 and 6, each blade holderconsists of two like parts 600 and 602 which are attached to theinsulating rings by means of screws. The inner faces 604 and 606 holdthe blade in place and are held together also by means of screws.

It can be appreciated, from the particular construction shown, that eachedge is near either the center post 300 on one of the rings 314 and 318,and that since the latter three metal members are connected together,the rings 314 and 318, as well as the center post 300, serve as thecounter electrode.

It will further be appreciated by one skilled in the art that althoughrazor blades have been delineated as the emitters per se, any comparablemeans can be used so long as each emitter operates in the field-emittingmode and no ionization of the liquid occurs.

What is claimed is:

1. In an EHD generator of the type having a injector assembly forinjecting charge carriers into a flowing liquid dielectric, a collectorfor scavenging the charges from said flowing liquid and means forrecirculating said liquid through the injector assembly and saidcollector, the improvement comprising:

an electrically conductive extension electrically in contact with saidcollector and extending from said collector to said injector assemblywithout touching said injector assembly,

said extension being attached to said collector and forming a partthereof, said extension being shaped to provide electric field lines offorce which, in the area near the output side of the in jector, aredirected radially inward when said collector and injector assemblies areprovided with potentials in the conventional manner for operating an EHDgenerator, and wherein said extension is a hollow tubular member whichis flared outwardly adjacent said injector assembly to define a mouthwith a cross-sectional area greater than the cross-sectional area of theoutput side of said emitter.

2. The invention as claimed in claim 1 wherein the mouth of saidextension is at least even with the output side of said injectorassembly so that there is no interspace region between injector assemblyand collector which is not surrounded by said extension.

3. The invention as claimed in claim 2 further comprising means foradjusting the relative positions of said collector mouth and saidinjector assembly output side.

4. The invention as claimed in claim 3 wherein said adjusting meanscomprises an elevator mechanism attached to said injector assembly.

I i i l

1. In an EHD generator of the type having a injector assembly forinjecting charge carriers into a flowing liquid dielectric, a collectorfor scavenging the charges from said flowing liquid and means forrecirculating said liquid through the injector assembly and saidcollector, the improvement comprising: an electrically conductiveextension electrically in contact with said collector and extending fromsaid collector to said injector assembly without touching said injectorassembly, said extension being attached to said collector and forming apart thereof, said extension being shaped to provide electric fieldlines of force which, in the area near the output side of the injector,are directed radially inward when said collector and injector assembliesare provided with potentials in the conventional manner for operating anEHD generator, and wherein said extension is a hollow tubular memberwhich is flared outwardly adjacent said injector assembly to define amouth with a cross-sectional area greater than the crosssectional areaof the output side of said emitter.
 2. The invention as claimed in claim1 wherein the mouth of said extension is at least even with the outputside of said injector assembly so that there is no interspace regionbetween injector assembly and collector which is not surrounded by saidextension.
 3. The invention as claimed in claim 2 further comprisingmeans for adjusting the relative positions of said collector mouth andsaid injector assembly output side.
 4. The invention as claimed in claim3 wherein said adjusting means comprises an elevator mechanism attachedto said injector assembly.